1. Field of the Invention
The present invention relates generally to wireless communications devices. More specifically, the present invention relates to techniques for tracking timing references in signals received by wireless communications devices.
2. Description of the Related Art
Mobile communications devices, such as wireless phones, are often employed in vehicles or other mobile platforms that travel at relatively high velocities. For example, mobile communications devices are often used in automobiles, trains, and airplanes that travel at speeds as fast as several hundred kilometers per hour.
In wireless communications environments, radio frequency (RF) signals are often transmitted between a transmitter and a receiver across a fading channel. When the transmitter and the receiver have a relative velocity with respect to each other, signals received from the fading channel exhibit fluctuations in amplitude and phase. These fluctuations have a peak rate that is a function of this relative velocity. According to this function, as the relative velocity increases, this peak rate also increases.
Signals sent across such fading channels may arrive at the receiver as a number of multipath transmission components. Multipath transmission components are individual RF wavefronts that stem from a single transmission by the transmitter. Multipath transmission components may be caused by wavefront reflections as well as by other physical occurrences. To a receiver, each multipath component appears identical, except for a distinct time shift, and usually a distinct signal strength and a distinct carrier phase.
Many receivers, such as ones employing coherent code division multiple access (CDMA) demodulation, require precise timing references in order to convert received signals into information symbols. These timing references are synchronized to modulation parameters that are associated with the received signals.
For mobile communications devices, such as wireless CDMA telephones, the reception of multiple multipath signal components from a fading channel causes difficulties in maintaining such precise timing references.
A CDMA receiver can distinctly identify and demodulate multipath components that are separated in time by durations greater than or equal to approximately 1.5 PN chips. To perform such identification and demodulation, receivers typically employ functional components called demodulation fingers. However, receivers are typically unable to distinctly identify and demodulate multipath components separated in time by less than approximately 1.5 PN chips. Thus, for such multipath components, a single demodulation finger must do its best to derive a time reference from multiple multipath components that provides optimal demodulation properties.
The maintenance of such derived time references is problematic because as fluctuation rates associated with the fading channel vary, so does the optimal time reference.
Accordingly, what is needed is a system and method that provides velocity responsive tracking of timing references in signals received by a wireless communications device.